A late season leaf disease of bur oaks

Oak leaf with significant late season leaf disease - causing leaf browning and defoliation.

Across southern Minnesota, a late season leaf disease can be seen on bur oaks, usually after August 1st. The causal fungus, Actinopelte dryina, has had a recent name change to Tubakia dryina. This year in August, the University of Minnesota Lab examined samples and confirmed that Tubakia is the main leaf spot fungus causing the foliage symptoms.

The appearance can be very dramatic as the entire crown turns brown except a few leaves at the very top. Defoliation can reach 90% in a few short weeks and affected trees look nearly dead. Late season defoliation has minimal impact on the tree?s health. However, several consecutive years of defoliation of this nature may have long-term impacts. Stored food reserves could be depleted resulting in dieback by insect and diseases of secondary action. This has not been observed to date on infected trees.

Large bur oak with substantial defoliation due to late season leaf disease.

Symptoms:

During late July and August, bur oaks scattered throughout southern and southeastern Minnesota exhibit the following symptoms:

  • Leaves with multiple leaf spots that eventually turn brown.
  • Brown or curled leaves most commonly in the lower crown. The infection progresses from the lower crown into the upper crown.
  • Whole tree crowns can become brown with curled leaves.